Porous silicones with open or intercommunicating pores are very attractive candidates for forming semi-permeable membranes, e.g., artificial biological membranes, due to the biocompatibility and good mechanical properties (i.e. elasticity and flexibility properties and tolerance to mechanical stretching) of silicones. For application as semi-permeable biological membranes, porous silicones with appropriately sized pores are highly desirable. There is a need for biocompatible membranes with pores that are small enough to prevent living cells from penetrating through the membrane, but large enough to allow diffusion of oxygen and nutrients through the membrane to keep the cells alive. Therefore, there is also a need for a manufacturing process for porous silicones with a controllable pore size and preferably with a limited variation of the pore size over the porous silicone. To allow penetration of cells through a porous membrane, the pores need to be interconnected in a thickness direction of the membrane.
Several attempts have been made to make porous silicones with controllable pore sizes. Various sacrificial fillers (both solids and liquids) can be used as porogens (pore generating agents) for manufacturing porous silicones. For example, U.S. Pat. No. 6,900,055 describes a method for making a porous silicone using solid porogens. When using liquid porogens (such as, e.g., water), a method for making porous silicones involves an intermediate step of preparing a water-in-silicone emulsion. This intermediate step of emulsion formation is challenging, because such emulsion is unstable and water tends to accumulate as a separate layer.
The stability of, e.g., water-in-silicone emulsions can be enhanced by using surfactants or emulsifiers. Both inorganic and organic emulsifiers can be used. Emulsifiers or surfactants reduce the surface tension of a liquid and lower the interfacial tension between two liquids, making an emulsion more stable.
In U.S. Pat. No. 5,362,761 a fabrication method is described wherein a silicone emulsion comprising a water emulsion of an organopolysiloxane, a cross-linking agent, a curing catalyst and an emulsifier is frozen and wherein the ice contained in the frozen silicone emulsion is sublimed (in vacuum) without being defrosted, such that a porous silicone with intercommunicating pores is obtained.
The approaches described in the prior art use at least one emulsifier or emulsifying agent whenever a liquid is used as a porogen. However, the majority of emulsifiers are toxic in varying ranges of concentration, which renders them unsuitable for prolonged contact with living systems. When a solid is used as a porogen, there is often a need for adding dispersing agents to avoid aggregation of micro-particles or nano-particles.
Therefore a complete elimination of emulsifiers and/or dispersing agents from the fabrication process of porous silicones is highly desirable for applications where the porous silicones come in direct contact with living cells or, e.g., with body fluids.